Switching to kevlar tiedown cables
Why do we need tiedowns:
Most telescope pointing errors are repeatable. These
include beam deflections caused by the motion of the azimuth
and dome. Using a pointing model (made by tracking sources), we
can get the pointing error of the telescope down to about 5 arc
seconds rms. This is needed since the FWHM beam at xband is 25
A 5 arcsecond error is a motion of 3 mm at the
feed. The repeatability of the errors allows us to position the
horns in absolute space to about 3mm.
The one pointing error that is not repeatable
is temperature. When the main cables heat up, they expand, when
they cool off ,they contract. During the night times, the
telescope will move vertically 1 inch for every 5 degree
To get the 5 arcsecond pointing rms we need to
remove the temperature variability. This is done using laser rangers
and an active tiedown system
- Note: We currently measure the air temperature. The thermal
motion of the cables depends on the cable temperature. At night,
the cable and air temperature are in equilibrium. During the day
the cable temperature can be 10 to 20 degrees above the air
temperature (at least thats what jim gould from amahan and
whitney has mentioned). So when i quote numbers for
temperature dependence, they will be valid for nighttime,
and for some hours of daytime.
The tiedown system:
The tiedown system controls the average height of
the platform. The system consists of:
- 3 jacks located under the dish below each platform corner.
- Each tiedown jack is connected to the platform corner above it
with a 1.5 inch wire rope (about 500 feet long).
- The jacks (under computer control) can apply up the
120000 lbs of tension on each corner of the platform.
- A laser ranging (distomat) system measures the average
- 6 distomats around the edge of the dish measure the distance
to the platform corners every two minutes.
- A tiedown correction is computer from the measured
platform height and then sent to the tiedown computers to move
Constraints in using the tiedown system.
- The required platforn height is fixed by the optics.
- The jack throw is limited to about 20 inches.
- The tiedown cables stretch as they are pulled: 1.7 inch
tiedown motion gives 1 inch of platform motion. So the 22
inches of jack throw is about 11.8 inches of platform motion.
- temperature variation has 5 degF moving the platform 1 inch.
The 11.8 inches of platform motion maps to 60 degF of
- If the temperature goes down, the main cables decrease in
length, so the platform rises. We then pull on the tiedowns.
- The maximum tension we can apply from one tiedown is 120kips
(1 kip = 1000 lbs).
- The maximum total we can apply from all tiedowns combined is
- If the temperature goes up, the main cables get longer and the
platform sinks. We release tension in the tiedowns to move the
platform back up.
- If we ever lose tension in a tiedown cable, then we loose
the ability to move the platform up (you can't push on a wet
- We run an unbalanced system (see
platform weight distribution .gif)
- The dome weighs 220 kips.
- The carriage house (on the opposite side of the azimuth arm)
weights 35kips. When using the dome we leave the carriage
house parked at 8.8 degrees za.
- There is a fixed counterweight of 45 kips at the end
of the carriage house side of the azimuth arm.
- The tension in a tiedown cable can change if the azimuth
points at the tiedown (it goes as cos(az - azTiedown)), and then
the dome moves in za.
- If the dome moves to a high enough za, we will lose tension
in a tiedown cable (even if all of the other tiedown cable
- This is the major reason why we lose tension in a tiedown
- If the platform is lighter (higher) then this problem occurs
Problems with using the tiedown system:
- The tiedown cables are the main path to ground for the
tower/platform. Lightning striking the towers/platform goes
through the tiedown cables when going to ground (see a partial list of tiedown failures.)
- When lightning passes through the tiedown cables, the
electrical equipement (cpu, i/o boards, motor power supply and
ampiflier, encoders, load cell amplifiers) can be damaged.
Lightning is a frequent occurrence at arecibo may through
- The cpu and i/o boards used for the tiedowns are no longer
- We recently ran out of cpu boards after a lightning
strike in oct11 destroyed 3 cpus and multiple i/o boards. It
took us 3 months before we found a vendor who made a one
time run to build cpu (30) and some i/o boards. We now
(2012) have a good supply of cpus and most i/o boards. There
is one i/o board that the vendor could not build. We only
have 5 spares for this one.
- Even with the spares, the lightning strikes require a lot of
man hours to debug and replace the components of the system
(it's not always as easy as ...replace the chips with the
holes in them).
Replacing the tiedown cables with kevlar cables.
A proposal was made to replace the wire rope
tiedown cables with kevlar cables. The benefits of this are:
- Kevlar is an insulator. This would stop the lightning from
using the tiedown cables as the path to ground.
- It's' been mentioned that a wet kevlar cable would still
conduct electricity through the water on the outside.
- The cables themselves will have to be protected from the UV
by some material. We currently use a plastic cover on the
linefeed guy wires which are kevlar (The plastic had been
replace a few times over the years they've been up there).
- A lightning pulse lasts for 10-20 microseconds with 20-30
kilioAmp currents and KiloVolt voltages.
- A thin layer of water on a plastic coat may conduct
electricity, but i doubt it could support any current before
the water is vaporized. As long at the air around the cable is
not ionized, there should not be an appreciable current flow.
- There are multiple electrical paths to ground from the
platform/towers (ground cable from tower to earth, cement
towers with rebar, power cables from platform). Currently the
1.5 inch tiedown cables are the lowest resistance path to
ground. By switching to a wet kevlar cable, it doubt
that this would still be the case (ok.. then we have to worry
about what the lightning does on the other paths to ground).
- Kevlar cables would weigh 11000 lbs less than the current
tiedown cables. If you think of the tiedown cables as part of
the platform, then the platform would weigh 11000 lbs less.
- For the same cold temperature, you would now have to pull
with 11000 more pounds (11000/3 per tiedown) to put the
platform into focus.
- Platform motion with weight:
- When there is no tension in the tiedowns, the platform
will move 1 inch when 8000 lbs are added to it.
- 11000 lbs would move the platform 1.4 Inches
(about the same as 7 deg F).
- When the tiedowns still have tension, it takes 24000 lbs
to move the platform 1 inch.
- In this case, the platform moves down and loses some of
the tension that was in the tiedown cables. You need
the 16000 extra lbs to make up for this loss.
- In this case 11000 lbs would move the platform up about
.5 inches. The same as 2.5 degF drop in temperature.
- When the dome goes up to a high za and points near a
tiedown, we would now have about 4000 more lbs of tension
before we would lose tension in a tiedown.
- For za motion of
dome (with td tension), the platform corner moves .46
inches for 1 degree of dome za motion.
- for no td tension,11 kips gives us 1.4 platform
inchs, or about 3 extra za degrees of motion. 2 za
- For td tension 11 kips gives about 1 extra degree of za
range for the dome.
- How often do we see lose tension in a tiedown.
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